I looked into trying to improve the 28 a few years back but came up blank..The leather washer had been replaced and was well lubed in what smelt like linseed which was probably ideal, while the fits of the piston and guide rod of good quality typical of firearm quality build pistols of this type ....Webley Seniors etc..The mainspring was also in good shape so I just smoothed out the action and cleaned out any grease, replacing with a drop of pellgun and a slim slip ring over the guide.At the club I put around 20 clearing shots through it and handed it back to my friend as apart tuned accurized 28 delivering 352 fps avg.No better ..I'm pretty sure the barrel is just a tad too short to get any real accuracy going, while the trigger completely detaches from the action for cocking which means it's never going to match say a well lubed Webley trigger .177 I got 2.5 inch groups and occasional flyers at 10 meters. Which is not quite what I make with a Webley Junior with adjustable rear sight but smooth bore barrel.Beautiful hefting pistol though....maybe best ever for feel ...Maybe I'll look at it further ....but at the moment trounced for performance by the HW70 we have....1.2 inch groups off hand and 430 fpsThe 70s barrel seems to accept pellets easier and has a lovely trigger and slightly greater size internalsI am going to look at major tuning of both but it needs new components really.I want to try to exceed 450 fps with the 70 but think this too optimistic and some for the 28

Having just acquired a Haenel 28R this is what I immediately noticed. This pistol feels like a fine firearm which through the ingenuity of Hugo Schmeisser happens to be an air pistol. This was made at a time when there was no 10 meter air competitions. Doubt there was ever a real goal for pellet speed. But Schmeisser developed one of the most unique cocking mechanics, one that would maintain the Luger feel and look. Don't think he was hung up on speed. What you have is a kind of mechanical masterpiece of a specific time and place. I have a FWB 65 which is a much better pistol but it made huge sacrifices to a firearms look and feel to get there. It's a classics of its own kind. But the 28 will be the last one I will probably give up.

Ok up-date ....you cannot do much to this 28 ....the mainspring is pressumably quite weak to facilitate loading with a beautiful but nonetheless inefficient design for mechanical advantage, with anymore compression likely to wreck the action or your wrist.I did find that running up a PTFE head with slim thickness added a bit of volume and improved efficiency but only managed 365 fps high or around 20 fps.However, by remote chance found that Falcon pellets transformed the gun. The barrel is quite tight but the Falcons very soft and slightly smaller in the skirt diameter than other similar brands.Result ....slight improved accuracy and another 20 fps improvement.

I did manage to hit 470 fps with the hw70 using Jsb match and slip ring....slight head adjustment

Thanks for your update and thoughts. I've had my 28r for a couple of days now and it is in great shape. But mine probably has the original spring in it and to be honest it must be weaken with age cause mine is not that hard to cock. Pushing round balls out at 250 or less. Personally I am happy with this given I am using it as a fun gun inside. The trigger takes a good pull to release. It is accurate enough to have fun but is never going to be a 10 meter pistol. I like the relative ease of cocking and the idea that all this means less stress on a pretty collectible old gun. The repeating action is kind of a hoot, easy to loose track as you run out. I have a 25 acp Haenel automatic pistol from this era, there is no difference in the milling and quality of finish, between air gun and firearm. 2.5 lbs of old school gun makers art, never will be done again.